The spine is the axis of the skeleton on which all of the body parts hang. In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar spine sits upon the sacrum, which then attaches to the pelvis, and in turn is supported by the hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints but allow known degrees of flexion, extension, lateral bending, and axial rotation and translation.
Typical vertebra has a thick anterior bone mass called the vertebral body, with a neural (vertebral) arch that arises from the posterior surface of the vertebral body. The central of adjacent vertebrae are supported by intervertebral discs. The spinal disc and/or vertebral bodies may be displaced or damaged due to trauma, disease, degenerative defects, or wear over an extended period of time. One result of this displacement or damage to a spinal disc or vertebral body may be chronic back pain. In many cases, to alleviate back pain from degenerated of herniated discs, the disc is removed along with all or part of at least one neighboring vertebrae and is replaced by an implant that promotes fusion of the remaining bony anatomy.
The success or failure of spinal fusion may depend upon several factors. For instance the spacer or implant or cage used to fill the space left by the removed disc and bony anatomy must be sufficiently strong to support the spine under a wide range of loading conditions. The spacer should also be configured so that it likely to remain in place once it has been positioned in the spine by the surgeon. Additionally the material used for the spacer should be biocompatible material and should have a configured that promotes bony ingrowth.
Corpectomy cages have been developed to help support the spine and maintain the normal spacing between opposing vertebrae. Typically, corpectomy cages are pre-manufactured at various heights requiring that a cavity between opposing vertebrae be prepared and distracted to a dimension corresponding to the most suitably sized corpectomy cage. The surgical procedure to prepare the implant site can be difficult and lengthy. Moreover, the procedure can increase risk of trauma to the tissues surrounding of the implant site.
Distractible corpectomy cages may be used as both a fusion device and/or a means for maintaining intervertebral spacing. Often these implants include a drive means that allows the corpectomy cage to be expanded in situ to a size that corresponds to the cavity created when the damaged tissue is removed. The drive means typically includes devices such as gears, threaded rods, and the like, in mechanical engagement so as to expand or contract the device to a necessary distance between the vertebrae.
Cervical cages are also used to stabilize the spine during the fusion process. These devices likewise include one or more plates and fasteners (typically screws) for aligning and holding vertebrae in a fixed position with respect to one another.
A common failure for corpectomy cages and cervical cages is the backing out of screws into soft tissues of the patient's body. The backing out of the screws is typically a result of the screws failure to achieve a sufficient purchase in the bone, although the stripping of the screws has also been known to cause this problem. Another common problem is that these devices require “carpentry” work to match fit aspects of the vertebral bodies.
Thus, it is important that the medical implant (corpectomy cage or cervical cage) is properly held in place by the fastener. To keep the fastener from backing out, a locking device is typically utilized. The locking device physically impedes the top of the screw from rising, which prevents the fastener from rotating (as this would cause the fastener to rise). Once the locking device is in the “locked” position, the fixation mechanism is held in place.
It is further important that the locking device be properly positionable both in the unlocked position (so that the medical practitioner can have access to properly position and implant the fastener) and the locked position (so that the locking device can properly function). Accordingly, there is a need for an improved locking device for corpectomy cages and cervical cages.